As a method for evaluation of the quality of a melt used for an aluminum alloy casting, the K-mold method is known. According to this method, a relatively small sample of the melt is taken on-site and cast in a K-mold. The fracture surface of the cast sample is observed by the measurer. By counting the number of oxides, pieces of film, and other nonmetallic inclusions, it is possible to quickly inspect the quality (Japanese Utility Model Publication (B2) No. 52-17449: hereinafter referred to as the “old measurement method”).
However, the measurer counts the number of nonmetallic inclusions present at the fracture surface by the naked eye or through a magnifying glass, so skill is required. There was therefore the defect that the measurement value differed depending on the measurer.
Therefore, the inventors developed a method of automatically counting the number of inclusions of a predetermined particle size (for example, 100 μm) or more by arranging a sample fracture surface at a special illumination system, using a CCD camera to capture an image of that fracture surface, processing the image for color density, and digitalizing it by predetermined threshold values so as to thereby eliminate the differences in measurement value depending on the measurer and enable easy measurement by a worker on site (Japanese Patent Publication (A) No. 2005-3510: hereinafter referred to as the “conventional automatic measurement method”).
However, in counting the number of inclusions by this conventional automatic measurement method as well, the image of the entire area of the sample fracture surface was captured, so even shadows due to fine shrinkage cavities present at the fracture surface ended up being captured in the image. For this reason, there have frequently been cases where the number of inclusions measured by a skilled worker and the number of inclusions measured by the conventional automatic measurement method have remarkably diverged.